A number of terms are used in this specification, the definition thereof are given as follows:
The term transparent or translucent sheet-like structure used herein refers to transparent paper, wax paper, or plastic sheets.
The term positive copy used herein refers to a copy with the words, pictures, lines, etc. thereon in their natural lights and shadows, not reversed; representative articles resemble slides which are generally known as positive films; however, the formation and features of the positive copy described herein are not the same as those of conventional slides.
The term negative copy used herein refers to a copy having lights and shades reversed, similar to developed negative films. A negative copy has to undergo the step of contact copying to obtain the same visual effect of the positive copy.
The term normal copy herein refers to a copy with words, pictures, etc., shown in their normal state after they are developed, e.g. "F" is shown as "F".
The term reverse copy used herein refers to a situation contrary to the above, i.e. "F" is shown as " ".
The term photosensitive roller herein refers to an embodiment of a photosensitive element which may be a roller or a plate.
Although relief printing, with type printing as a typical example, has a very long history, undeniably, there is a tendency that plate printing is going to replace conventional type printing. In particular, precoated or photosensitive (abbrev. herein as PS) plates have the advantages of better durability and surface quality in terms of eveness over conventional type printing in which types may drop out; surface is uneven, and printing speed slow.
At present, the process of making PS plates is still very complicated. First of all, the copy to be printed is photographed with the use of a special camera on a film which is then processed and developed to obtain a negative (negative copy).
Several negatives are prepared in this way. It then requires skilled workers to retouch them on a contact copying sheet placed on a retouching desk having a luminous board. The sheet is then made into a positive plate by means of a contact copying device. The positive plate may then be processed on a PS plate to obtain a reverse copy. Finally, the PS plate is processed and developed to obtain a printing plate ready to be mounted onto the printing machine.
From the above-described method using the PS plate as a representative example, it can be seen that the conventional method of plate making is very complicated. It involves at least six steps: photographing, film processing, retouching, contact copying, plate processing and developing. The whole process is very time-consuming; this is the first disadvantage of the conventional plate making process.
Another disadvantage is that photographing and film processing require professionals or skilled workers. Labor cost will be greatly increased if these people are lacking. If they resign or go on strike, the subsequent steps of the process may also be delayed.
A further disadvantage is the disposal of negatives and the chemical substances used in film processing. These chemicals not only endanger the health of the workers but also pollute the environment.
The conventional process of plate making has been widely used in the printing industry, but the above-mentioned three disadvantages have always existed. Improvement is therefore necessary.